Everything is Multiplicative. So if you have a Hot Shot and Trick Attack up on the same time, it's 1.1*1.05 = 1.155, rather than 1.15. As for the amount of "averaged" DPS a Ninjas Trick Attack provides to a player/raid, it's 1+0.1*10/60 = 1.01666666667, or 1.6667% Raid DPS.

In regards to gear, Dervy is actually doing a complete overhaul of his set. He's getting Augmented Gloves, Belt and Earrings swap once he gets the Twines/Coats and dropping to 558 SS (well, 558ss + ss from lore weap)